Traveling wave tubes



1956 J. H. BRYANT ET AL 2,771,565

TRAVELING WAVE TUBES Filed Aug. 19, 1952 A l A A A A A A A ATTORNEYTRAVELING WAVE TUBES John H. Bryant, Nutley, and Billy D. McNary,Pompton Lakes, N. J., assignors to International Telephone and TelegraphCorporation, a corporation of Maryland Application August 19, 1952,Serial No. 305,151

6 Claims. (Cl. SIS-3.5)

This invention relates to traveling wave electron discharge devices andmore particularly to means and methods for introducing a circuit lossalong the path of the R.-F. field of a traveling wave in such devices toprevent undesirable oscillations therein.

The traveling wave type of electron discharge device or tube isparticularly useful in wide band microwave systems since it is capableof amplifying radio frequency energy over a very wide band offrequencies. The tube includes a form of transmission line, usually ahelix, for transmission of microwave energy for interaction with anelectron beam closely associated with the line. The helicalcharacteristics of the transmission line are such that the axialvelocity of microwave signals conducted along the helical path isapproximately the same as or slightly slower than the velocity of theelectrons of the beam, whereby the electric field of the microwavesignals interacts with the electron beam for amplification of themicrowave signals.

Traveling wave amplifier tubes heretofore proposed usually have anelectron gun and a long, slender, glassenclosed radio frequency sectionwherein the interaction occurs. The radio frequency section includes aninput connection for the R.-F. energy at or immediately adjacent to theoutput of the gun and an output connection at the other end of thesection adjacent to an electron collector electrode.

In the employment of this type of tube, the useful range ofamplification which can be utilized is limited by a tendency to generateself-sustaining oscillations as the .amplification is increased. Thiseffect is usually due to mismatch between the output circuit of thedevice and the load circuit over all or part of the wide range offrequencies to be amplified. Due to such mismatch, energy of at leastcertain frequencies is reflected back toward the input end of theamplifying device. When the reflected wave is not attenuated in itstravel along the 1 helix in a direction opposite to the motion of theelectron stream, some energy reaching the input end of the device isreflected from the input end causing the generation of self-sustainingoscillations. Thus, the energy reflected or transmitted back to theinput end must be attenuated if the tube is to remain stable.

In most traveling wave tubes heretofore proposed, attempts have beenmade to overcome this tendency of generating self-sustainingoscillations by employing resistive or lossy material to attenuate thereflected waves. While such proposals either distributed the resistivematerial along the entire length of the helix or along a major portionthereof, or in lump form disposed in spaced relation to the helicalconductor but in the electromagnetic field, or as a part of the helicalconductor itself, such provisions limited the gain and power output ofthe tube. Also, where the resistive material is spaced from theconductor, it is less effective and requires a larger amount in order toabsorb the reflected energy. Where the resistive material is provided inlump form, it is either spaced from the helical conductor or used as aUnited States Patent ice 2 terminating resistance at some radian on oneend of the conductor.

It is one of the objects of the present invention, therefore, to providean improved means for introducing a circuit loss in the path of theR.-F. field of a traveling wave tube so as to prevent self-sustainingoscillation and yet obtain high gain and maximum output power We havefound that to obtain high gain and maximum power output that theresistive material must be concentrated within the smallest axial lengthof the helix as possible and with proper impedance match, theconductivity of the helical conductor must be maintained high and theoutput section of the helix must not only be as loss-less as possiblebut as long as permissible. It is, therefore, another object of thisinvention to provide the wave propagating structure of a traveling wavetube with a body of resistive material in a concentrated body having therelationship to satisfy the above-mentioned requirements which We havefound desirable for high gain and maximum power output.

One of the features of this invention is the use of a loss producingmaterial of suitable resistivity to provide a structure which has partsthat taper radially at its ends relative the helix or other propagatingstructure of a traveling wave tube to intercept part of the highfrequency field with a minimum of reflection or radiation of the R.-F.energy and which produces an impedance transition with the R.-F. fieldin a minimum axial distance along the propagating structure.

A further feature of this invention is the method by which the lossymaterial, having uniform conductivity over large temperature variations,may be constructed and located in proximity to the propagating structureof the traveling wave tube. By spraying a colloidal graphite suspendedin an air hardening binder onto a form having the required shape, aproperly-shaped lossy structure is formed around the helix. After thebinder has set, the form is extracted with solvents leaving the requiredlossy structure intact.

The above-mentioned and other features and objects of this inventionwill become more apparent by reference to the following descriptiontaken in conjunction with the accompanyingdrawings, in which:

Fig. 1 is a longitudinal cros-sectional view partly in block form of atraveling wave tube in accordance with the principles of this invention;

Fig. 2 is an enlarged longitudinal cross-sectional view of the helicalstructure shown in Fig. 1;

Fig. 3 is a cross-sectional view taken along line 3--3 of Fig. 2;

Fig. 4 is a longitudinal cross-sectional view of an alternate embodimentof the lossy material structure of this invention;

Fig. 5 is a cross-sectional view taken along line 5-5 of Fig. 4;

Fig. 6 is a longitudinal cross-sectional view of another embodiment ofthis invention; and V Fig. 7 is a cross-sectional view taken along line77 of Fig. 6.

Referring to Fig. 1, there is shown an illustrative embodiment of adischarge device adapted to be used as an amplifier at ultra-highfrequencies. The arrangement shown comprises an electron beam tubeincluding an evacuated envelope having an elongated portion 1. Thisportion, which is of uniform diameter along its length, connects with alarge electrode-containing portion 2. The envelope 1 may be constitutedof a low loss insulating material, such as glass or quartz.

. The tubular envelope portion 1 is provided at one end with means, suchas a known type of electron gun 3, for producing an. electron beam orstream. The electron stream emerges from gun 3 and travels along a paththat is straight throughan input coupler 4 and axially down theevacuated tubular envelope 1. The electron'stream is furtherconcentrated and guided along an axial path within the space surroundedby a helix 5 by a magnetic field produced-by cylindrical coil 6.Electrode 7 serves to collect the electrons arriving at the end of theenvelope 1.

The helix 5, which serves as the path along which the wave may bepropagated, is wound with several turns per wavelength along its axis,which may preferably be of a length of 30 to 40 wavelengths of thefrequency to be amplified. The helix is supported by a series ofnonconductive rods 8 equally spaced around the circumference which maybe composed of a ceramic material and which are disposed between thehelix 5 and the envelope 1 and supported by discs 8a and 8b. The helix 5is joined to the input coupler 4 by the input impedance matching section9 and the output coupler 10 by the output impedance matching section 11.These matching sections are simply extensions of the helix in which thespacing between turns is increased along the circumference of the helixand act as tapered transmission lines to provide a wave transmissionpath of uniformly changing impedance from the relatively low impedanceat the end of the couplers 4 and 10 to the relatively high impedance ofthe central portion of the helix 5 with a minimum reflection of energyback to the signal source.

In order to utilize the device in an operable system, there is providedan incoming wave path represented by the dotted input waveguide 12 intowhich there is introduced the input wave signal to be amplified. Anoutput wave path shown as the output waveguide 13 serves to transfer theamplified output Wave to the load circuit. The wave from the inputwaveguide 12 and coupler 4 travels along the circumference of the helix5 at a speed approximating that of light, but at a linear velocity alongthe axis of the tube which is smaller in proportion to the ratio of thedistance between turns to the circumference per turn. As the beam andthe radio frequency wave travels along the helix, an interaction takesplace whereby energy is transferred from the beam to the wave therebygreatly amplifying the wave. As the amplified wave reaches the outputend of the helix 5, it is transferred to the output waveguide 13 bymeans of the output coupler 10. As the amplified wave reaches theimpedance matching section 11 at the output end of helix 5, even with anextremely favorable termination, at a given band of frequencies, therewill still exist reflected waves at frequencies outside the given bandat the output end of the helix. This wave is very little affected by theelectron stream and hence will propagate back along the helix 5 towardthe input end with attenuation. The reflected wave will reach the inputend of the helix 5 with attenuation equal to the circuit attenuation andwill in turn be reflected back toward the output end of the helix. It isobvious that there will be some reflected energy which will result inself-sustaining oscillations, provided there is not enough circuitattenuation to dampen the reflected energy. It will thus be seen thatthe deliberate introduction of an artificial loss along the helix so asto provide a dissipation of the reflected wave will serve to greatlyincrease the range of useful amplification which can be achieved with adevice of this type.

In accordance with the principles of this invention, the artificial lossis introduced along the portion of the helix by a structure 14 of asuitable dissipative material, such as graphite. Referring to Figs. 2and 3, wherein one type of structure found particularly suitable isshown, it is seen that the lossy material structure 14 is thickest atits center where it is closest to the helix 5, and it has parts that aretapered ralially outward toward each end. The center of the lossystructure is in close proximity to and preferably actually touching thehelix 5. When the lossy structure 14 is in direct contact with thehelix, the attenuation introduced into the circuit will be greatest. Theloss-producing structure 14 is supported by the dielectric supportingmembers 8 of the R.F. propagating structure and is situated between andadjacent to the supports. The location of the lossy material around thepropagating structure of the traveling wave tube permits theinterception of part of the high frequency field associated with theelectromagnetic energy being propagated on the helix and thereforeattenuating the electromagnetic wave while the tapering of the/endsprovides a gradual transition with substantially no reflection of theR.-F. energy. The disposition of the loss is generally such that most ofthe electromagnetic energy in the propagating structure is removed, andthe signal is transmitted through the loss section by the modulationsignal energy in the electron stream. At or near the output end of theloss section, the electromagnetic wave is re-excited by the modulatedelectron beam. The location of the lossy material is a predetermineddistance from the input end such that the equation CN=-.25, where C isthe coupling parameter relating the degree of interaction between theelectron beam and electromagnetic wave propagated along the propagatingstructure of thhe traveling wave tube and N is the number of wavelengthsfrom the input end. Pro vision of an input portion of helix having alength corresponding to CN=-.25 insures that the electron stream becomessufliciently excited according to the R.-F. signal from input 12 that anelectromagnetic wave is re-excited beyond the loss section in the outputportion of the helix. This rc-excited wave in turn interacts with themodulated beam in a continuous manner as the wave and the beam progressdown the tube at practically the same velocity in such a manner that theelectromagnetic wave gains in amplitude. Providing a length of inputportion of helix greater than that corresponding to CN=-.25 would result in little or no added gain if the attenuation of theelectromagnetic wave by the lossy material is high enough to remove mostof the electromagnetic energy.

One method of constructing the lossy material structure, that we havefound satisfactory, comprises the steps of first preparing the helixassembly to insure its cleanliness and freedom from oxides, acids, andsalts. The dielectric supporting rods 8 are equally spaced around thehelix 5 so that the magnitude of loss desired may be easily reproduced.The helix 5 is then painted with a thin film of organic plastic, such asnitrocellulose containing the necessary solvents and plasticizers, togive a thin, tough cylindrical coating over the helical structure whenthe plastic is dry. Care must be exercised that the cylindrlcal filmdoes not contain holes between turns of the helix wire and that auniform coating is obtained which makes contacts with supporting rods 8but which does not extend up the sides of the rods 8. The helicalstructure, or a desired part thereof, is then dipped in or otherwisesubjected to molten parafiin or other similar material, and since thehelical structure is cooler than the paraffin at its melting point, theparaffin adheres in a uniform coating over the helix. The coat of wax isbuilt up by repeated applications until a paraffin wax form equalsubstantially to the outside diameter of the supporting rods isobtained. The wax, for example, may also be applied with a brush or asmooth glass rod. The wax is then scraped or molded into a bell-shapedtaper down to the organic film. The nitrocellulose film can also betapered or removed so that turns of the helix are exposed to contact thelossy material by using a smooth glass bead which has been dipped inacetone. Rubbing the acetone over the dry organic film will dissolve thesurface layers, and thus the thin lacquer film can be removed. Colloidalgraphite in a binder is then sprayed or otherwise applied onto theprepared form around the helical structure. After the binder has set,the wax is extracted by using carbon tetrachloride or other suitablesolvent, andnitrocellulose film being extracted with acetone or othersuitable solvent. Although it is not essential, the structure can be airfired at 375 C. after the wax form is removed and may then be vacuumfired at 800 C. to remove gases from the lossy material and other partsof the tube. When it is necessary to place the loss structure in contactwith the helix to produce the required amount of attenuation in theshortest possible axial length of helix, the degree of electricalcontact must remain constant through large variations in the temperatureof both the helix and the energy absorbing loss structure. Since thethermal expansion coclficients of the helix material and of suitableloss material cannot be exactly matched and since a temperature gradientof unpredictable magnitude will exist between the helix and the lossstructure, it is desirable to bind particles of loss material ofmicroscopic size in definite contact with the surface of the helix wire.A slightly flexible binder is required to hold the particles of lossmaterial in contact with the helix wire and in contact with each other.As shown by the above steps, the requirements on the binder for thecolloidal graphite are extremely severe since it must mix with graphite,be adaptable to spraying, must wet wax, collodian, and glasssufficiently well to start a uniform film. The binder must set by dryingin air adequately at room temperature within a reasonable time towithstand the action of hot carbon tetrachloride and acetone and mustthen harden when baked. Shrinkage must be low enough that the lossymaterial is not pulled away from the helix or develop cracks anddiscontinuities in the surface thereof. We have found that the silicateswith their high melting and decomposition temperatures are the bestchoice as a binder, particularly sodium silicate which we have found tohave the most desirable properties for fabrication.

With the method described above, it is possible to taper the lossymaterial structure as near to the helix and in any profile required togive the proper impedance match. Such a structure has stable electricalproperties with respect to temperature and is able to withstand theusual mechanical vibration tests.

Referring to Figs. 4 and 5, an alternate embodiment of a lossy materialstructure in accordance with the principles of this invention is shownwherein a graphite block 15 is constructed to fit into and around thespace between the dielectric supporting members 8 of the propagatingstructure 5. Since the field is confined to a region within an extremelysmall part of a wavelength from the helical transmission line in orderto produce the magnitude of attenuation necessary to preventself-oscillation, we have found that the graphite block 15 must besituated in extremely close proximity to the helix 5. The closetolerances required and the necessary taper are obtained by machiningthe graphite blocks to fit the specifications of the helix andsupporting rods. Graphite appears to have the most desirable electricaland vacuum properties than any material used for RAF. loss byabsorption. The electrical properties of graphite are sensitive tometallic impurities, and for reproducible results we have found itdesirable to use spectroscopically pure graphite produced artificiallyby electrolytic methods.

Referring to Figs. 6 and 7, another embodiment of a lossy materialstructure in accordance with the principles of this invention is shownfor use with a traveling wave electron discharge device of the hollowelectron beam type wherein said lossy material structurae 16 is axiallydisposed within the propagating structure 17. The propagating structure17 is supported on rods 18. The lossy material structure 16, disposedbetween support rods 18, is tapered on both ends away from thepropagating structure 17 and at its center is in close juxtaposedrelation with or touching the helix 17. The lossy material structure 16may be formed and then slid over support rods 18 inside the helix 17,during the construction of the propagating structure.

While we have described above the principles of our invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by Way of example and not as a limitationto the scope of our invention as set forth in the objects thereof and inthe accompanying claims.

We claim:

1. In a traveling wave electron discharge device having a conductor inthe form of a helix for transmission of radio frequency energy and meansto project a beam of electrons parallel to the axis of said helix havinginteraction with the electromagnetic field of the radio frequency energytransmitted by said conductor; an attenuator disposed along certain ofthe turns of said helix, said attenuator comprising a concentrated bodyof resistive material having its central portion only in direct contactwith at least one of said certain turns for maximum attenuation andspaced relative to other of said certain turns for impedance transition,said helical conductor being supported by a plurality of dielectric rodsdisposed parallel to the axis of said helix and said portion of saidbody being disposed transversely between at least two of said rods.

2. In a traveling wave electron discharge device according to claim 1,wherein said dielectric rods are disposed along the outside of saidhelix and said body of resistive material surrounds said helix and rodassembly and said portion extends radially inwardly between two of saidrods.

3. In a traveling wave electron discharge device according to claim 1,wherein said rods are disposed on the inside of said helix and said bodyis disposed within said helix with said portion extending radiallyoutwardly between two of said rods.

4. A method of forming an attenuator in operative relation with respectto the helical conductor of a traveling wave electron discharge devicecomprising the steps of assembling in association with a part of saidhelix a body of material conformed to the configuration of said helix,one part of said material being electrically resistive and non-solublein certain dissolvents while other of said material is soluble andremoving said soluble part by subjecting the assembly to a dissolvent,said soluble part being first applied to form a body about said helix,selectively removing a portion of said body adjacent certain of theturns of said helix, applying onto said body a coating of saidnon-soluble resistive material whereby when said soluble part is removedthe resistive material is disposed in desired relation to the turns ofsaid helix, said resistive material comprises graphite and a binder ofsilicate.

5. A method of forming an attenuator according to claim 4, wherein thesoluble part includes a wax material and said certain dissolventcomprises carbon tetrachloride.

6. A method of forming an attenuator according to claim 4, wherein saidsoluble part includes a first layer of organic plastic ofnitro-cellulose and a second layer of parafiin, and said certaindissolvent includes carbon tetrachloride for removing the wax andacetone for removing the organic plastic.

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